Existing commercial processes for removing sugar from sugar cane may be classified generally as tandem mill processes or diffusion processes. In tandem mill processes, the fiberized cane is repeatedly subjected to high pressure, usually in the range of 200 to 300 kg/cm.sup.2 in order to separate the sugar juice from the cane. In diffusion processes, solvent is allowed to percolate through a bed of fiberized cane by gravitational flow to extract soluble substances like sugar solution by lixiviation. Many modifications of the foregoing processes have been made. For example, the imbibition liquid is ordinarily utilized in mill tandem processes; and with it, maceration has been applied to a limited extent to improve extraction efficiency.
Basically, the improvements in mill tandem processes have been made in direction of using equipment able to exert increasingly higher pressures on the cane with the objective to obtain improved extraction results. Such measures have resulted in increased power requirements accompanied with increased costs for maintenance and overall operational expenses.
The high pressure utilized in conventional mill tandems allows for only a small maceration rate due to the fact that juice extraction on the front end of a mill tandem utilizing high pressure restricts the amount of maceration. If higher maceration rates could be possible in such mill tandems, the poor drainage capacity of a conventional 3-roller mill would be restrictive.
A moisture content in bagasse higher than 70% by weight creates severe feeding problems and consequently impairment of mill performance. To overcome this deficiency, many devices have been developed in the past in an attempt to improve the feeding of wet bagasse to high pressure mills but with only limited success. For this reason, tandem mill processes using high pressure and utilizing force-feeding devices are making such equipment only applicable to fiberous materials having a relatively low moisture content.
In addition to technological problems explained above, any increase of the volume of solvents must lead to an undesirable dilution of the extraction fluid leaving the process, which in turn must cause an increase of the energy consumption for thermophysical processes as required for sugar or alcohol production.
Sugar cane harvesting methods are tending toward more utilization of mechanical equipment, which in turn is carrying more foreign matter into the extraction process causing increased wear and with it increasing maintenance costs, particularly when high pressure mills are in use. Under such circumstances, the average performance of a mill tandem in view of capacity, extraction, and overall operation costs can be substantially impaired.
During the last 2 decades, diffusion processes have been devised with the objective to replace mill tandems with their obvious technological deficiencies. Diffusion processes require comparatively more complex and expensive equipment than the mill tandem processes; and, therefore, they have proven to be particularly feasible for sugar extraction plants having high manufacturing capacities. Extraction efficiency obtained with diffusion processes can be higher than for mill tandems; and, therefore, their implementation for many operations is justifiable.
Since lixiviation is the basic method in a diffusion system (diffusion is not the appropriate technical term), the cane must be specially prepared; and the extraction process is rather time consuming requiring 40 to 60 minutes. Heat is applied to the system, and the process temperature is ideally 76.degree. C. with the objective to accelerate lixivation and to prevent adverse bacteriological changes which may effect the solvents and fiberous material during the time period needed for the extraction process.
Nevertheless, exposure of all the materials involved is causing some destruction of the extractable solubles, inversion, and a drop in the pH: phenomenon which must be thermally and chemically controlled. Continuous operation of a diffusion system is essential; and interruption of this process, even for a short time period can cause a substantial impairment of the extraction result. The rate of lixiviation is closely related to the flow rate of the solvents through the fiberous material; and the respective percolation rate can be kept only on a high level when continuous operation is assured.